Products composed essentially of the .beta.-crystal form of CPC have achieved considerable importance as pigments because of outstanding properties with respect to stability, tint, and color intensity. Quite commonly, however, the attainment of such products has required the utilization of costly milling techniques, particularly when the more useful small particle sizes are desired. Thus, for example, a step of phase conversion may be typically necessary to ensure that all of the CPC is in the .beta.-form, yet it is also conductive to the growth of crystals to an excessively large size. Hence there has often been no alternative but to ball mill or similarly finish the resultant material.
A technique widely used for preparing CPC in small particle form involves dissolving crude CPC in sulfuric acid and reprecipitating it in a large volume of water under agitation. However, the product formed is .alpha.-CPC, which is characterized by a reddish tint and lack of stability. One modification of this technique, aimed at producing a stable CPC is described in British Pat. No. 824,558. According to the procedure described therein crude CPC is dissolved in a mixture of sulfuric acid with an aliphatic glycol in the presence of an aromatic hydrocarbon solvent and then reprecipitated by adding the solution to vigorously stirred water. The product so produced is more stable than the .alpha.-CPC produced by the unmodified technique described above, but is described as having the reddish tint characteristic of the .alpha.-phase and would therefore require an additional processing step for conversion to the .beta.-phase, characterized by a greenish shade.
A technique for the production of .beta.-CPC in a small particle size, i.e., of &gt; 40 m.sup.2 /gram, without the need for a milling step would accordingly be highly attractive.